SHM - Structural Health Monitoring – Canary in the Coalmine
Canaries were used as toxic gas detectors in mines as late as 1986, when electronic detectors replaced them. In much the same way, strain gauges are used more and more in structures for similar reasons of safety and predictability.
All structures degrade – it’s an inevitable result of aging and environmental effects. Corrosion, vibration, settlement, extreme events such as weather or impact, loads beyond the design criteria, or poor design, material choice or implementation during construction can all cause failure. As a minimum this can cause severe disruption (closing a bridge can cause miles of diversion, hours of lost time and huge financial implications) or, in the worst case, loss of life. Recent well-known and reported failures make this all too real for those directly affected (condominium in Florida, USA, road bridge in Genoa, Italy and the Florida International University pedestrian bridge collapse during installation in the USA are well-publicised examples).
Whilst failures cannot always be reliably predicted (extreme weather events, accidental vehicle impacts) there are many instances where failures are a result of poor maintenance or predictable structural degradation such as fatigue. Estimates of life based on typical load predictions can help, but in reality, conditions and loads change over time and therefore a reliable way is required to predict maintenance based on actual use and conditions as well as extreme events.
Strain gauges are a natural choice. Mounted in various locations around the structure, the data accurately represents the structural response and can be used to compare to the design and material limits, and fed into digital twins (computer models designed to run alongside physical objects and systems). For example, load distribution can change as cracks develop or foundations settle, bolt tension can be monitored predictively, wind loading can be used to close a bridge etc. Additionally, accidental or unpredicted conditions can be used to estimate the damage and whether usage needs to be modified or ceased for maintenance – the emergency closure of the existing Forth Road Bridge during the new Queensferry Crossing construction in Scotland, UK, is a great case study for this process and enabled the bridge to re-open in just a few weeks.
As developments in computer modelling, AI, digital twins, big data and IOT increase in capability there is a drive for the creation of smart structures that are manufactured with integrated sensors to improve safety, increase the design life, and reduce material usage thereby minimising the environmental impact of construction. Strain gauges can be integrated during design and construction, or added to existing structures using a variety of methods from custom-built sensors, adhesive-bonded or weldable gauges, or embedment gauges in concrete (discrete or installed on rebar). With an integrated approach in combination with modern and novel materials and construction methods including 3D printed structures, strain gauges are an essential tool in this revolution.